The Argentine pontiff is due to arrive in the Portuguese village on Friday before marking the 100th anniversary of the first reported apparition the next day by declaring two of the three children - siblings, Jacinta and Francisco - saints.
It is believed the mother of Jesus appeared in Fatima to them and their sister on six occasions between May and October 1917.
The third sibling, Lucia Dos Santos, who also allegedly witnessed the vision, became a nun and died at the age of 97 in 2005.
Fatima Shrine
Francis's deputy, Cardinal Pietro Parolin, said the importance of Fatima lies specifically in the fact that poor, illiterate children were able to convey a powerful message of love and forgiveness at a time of war when "the talk was of hatred, vendetta, hostilities".
Peter Williams with Catholic Voices said the occasion is monumental for the Catholic Church.
He told Premier's News Hour: "It is something which is an incredible worldwide movement, an incredible worldwide devotion.
"It's the 100 year anniversary so think that's why so many people are turning out to celebrate this. It's an incredibly special occasion".
The pope's visit to Portugal this week has drawn huge crowds so far and the BBC reports that more than a million pilgrims are expected to gather today for his pilgrimage.
He follows John Paul II and Benedict XVI (16th), who also made pilgrimages to the Fatima Sanctuary. Jacinta and Francisco were beatified by Pope John in 2000.
There will be a torch-lit procession on Friday evening and a Mass on Saturday.
Williams said the event will be a very spiritual for those taking part.
He said: "The idea of a pilgrimage is to go to a place that's been sanctified by a holy event and to just really spend time a lot of time with God, a lot of time in prayer, a lot of time at the holy sacrifice of the mass, a lot of time in confession, whatever it is that is going to be spiritually blessing.
"It's a time of concentration with God."
